<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>清除内外边距</title>
    <style>
        /* 这句话也是我们css 的第一行代码 */
       * {
           margin: 0;
           padding: 0;
       }
       span {
           background-color: pink;
           margin: 20px;
       }
    </style>
</head>
<body>
   123
   <ul>
       <li>abcd</li>
   </ul>
   <span>行内元素尽量只设置左右的内外边距</span>
</body>
</html>